Why is the emission shader clamping even when I increase the strength to crazy high values?
Mesh lights tend to be pretty noisy, especially if they are small. I hardly ever use only meshlights to illuminate a scene. If you put one of your lights closer to the road, you can see it does not get clamped, it's just very noisy. My suggestion is to switch to spot lights. Your lamp posts are 12m high, so to actually illuminate the street you do need a lot of power. I set the strength of spot to 20,000, that did the trick. If you are uncomfortable with such high numbers you can use a light falloff node to make the lamp ignore the distance.